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C.6 Time-domain EM methods

Introduction

Frequency domain electromagnetic methods detect near surface conductors through the

secondary magnetic fields that are induced by the primary magnetic field. With systemssuch as the EM31 and EM34 the secondary magnetic field can be 10-20% of the primarymagnetic field.

As the conductor becomes deeper, the secondary magnetic fields become weaker, and canbe difficult to detect in the presence of the much stronger primary magnetic field. Typicalsecondary magnetic fields are expressed as parts-per-million (ppm) of the primarymagnetic field. In this configuration, it is very difficult to measure the secondarymagnetic field in the frequency domain with a towed bird. Some success in this area has

been achieved with quadrature EM systems that measure the phase difference betweenthe primary and secondary magnetic fields. With up to 5 frequencies, this type of systemcan discriminate between good and bad conductors (for example McPhar Quadrem).

However most frequency-domain AEM systems use a rigid boom to detect the weaksecondary fields. This is the basis of systems such as DIGHEM and can use multiple coilgeometry and multiple frequencies.

Time-domain EM methods represent an alternative approach to detecting weak secondarymagnetic fields. This works by simply switching the primary field off and observing thedecay of the secondary magnetic fields.

This method is often referred to as transient electromagnetic exploration (TEM) or time-domain electromagnetic (TDEM) exploration. One of the first applications of TEM wasdescribed by Ward (1938). More details of the development of airborne EMinstrumentation can be found in Fountain (1998).

C.6.1 Basic physics

C6.1.1 Qualitative solution and the smoke rings

Direct electric current flows through the transmitter loop and generates a staticprimarymagnetic field (HP).


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The transmitter current is then switched offand the primary magnetic field immediatelyfalls to zero. This induces a secondary electric current in the Earth. The secondary currentacts to oppose the decrease in the primary magnetic field (Lenzs Law). The secondaryelectric current distribution can be approximated as a horizontal loop of current andgenerates a secondary magnetic field, HS (t).

Over time the secondary electric currents spread out (diffuse) in a pattern that is similar toa smoke ring. It moves deeper as time increases, and thus gives information about

progressively deeper structure. Initially the magnetic field is oriented downwards at the

RX.

As the current ring passes beneath the RX, the sign of HS changes sign.


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Nabighian (1979) made an analogy to a set of smoke-rings. An additional description ofthe physics and mathematics can be found in Hoversten and Morrison (1982). This

process can also be visualized as a contour plot of electric current density. Movie is basedon software written by Lynn Chotowetz in Physics 499.

Note that:

(1) The voltage (V) generated in the RX coil by changes in h is measured as a function

of time, where V= -

S

dt

tdhS )(

(2) Depth of EM signal penetration can be expressed in an analogous way to the skindepth that was used in frequency domain EM methods. One can show that in principle,the depth of penetration in a halfspace with conductivity at a time tcan be expressed as:

2

1

2

=

t

T

Comparison of coincident MT and TEM data led Meju (1996) and others to propose thatthe correct formula should be

3.2

TTEM

eff

=

See also Meju (1998) for further discussion of this topic

C6.1.2 Quantitative solution for a halfspace

A small transmitter (TX) loop is placed on the surface of the Earth at z=0 with the axis inthe z-direction. This comprises a vertical magnetic dipole. The receiver (RX) is locatedat (x,z). It can be shown that the time derivative of the vertical magnetic field at a time tafter switch of is given by:

++=

22)469)(2

()(9)2

( 44225

eerf

IA

t

hz

whereA = area of the transmitter loop;I= initial current in the transmitter loop


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222 zx += ;t4

= and erf denotes the error function. The receiver is located at a

distancex from the TX and an elevationz above the surface. More details can be found inTelford, Chapter 7.

The figure below shows )(tt

hz

for x=100 m and = 1 m and 1000 m

Note that at early time, )(tt

hz

decays at a constant rate. During this early time the

secondary current (smoke ring) is localized beneath the TX and has not passed underthe RX.

The secondary current flows in the Earth for a longer time if the ground is a goodconductor.


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As the concentration of secondary current (smoke ring) passes under the RX, the valuechanges sign, passing through zero (positive to negative). This causes the cuspobserved in the figure.

At late times )(tt

hz

decays with a relative simple form

2

52

3

20

)(

= t

IA

dt

tdhz

Thus at a given time (t) after switch off, the value ofdt

tdhz )( at late time will become

larger as the ground becomes more conductive.

Beware the log-log plot! If a linear axis is used for both )(tt

hz

and time, then the plot

only shows the late time. The change in sign occurs in the extreme left side of the plot

(not visible). Note that the )(tt

hz

decays slowly for the 1 m halfspace and quickly

for the 1000 m halfspace with the RX at 100 m from the TX.

Compare these results with those derivedfor INPUT in 6.2.1 where linear timeand voltage scales were used.


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C6.1.3 Quantitative solution for a layered Earth

In a layered Earth, the ring of electriccurrent will propagate downwards andoutward through the various layers. In a

low resistivity layer the decay will berelatively slow. In a high resistivitylayer, the decay will be relatively fast.

The example on the right is taken fromFitterman and Stewart (1986) and showsthe transient response of a two layerEarth. When the ratio of rho1/rho2 issmall (e.g. 1/16) the late time responseshows a relatively slow decay.

When the ratio of rho1/rho2 is large (e.g.16) the late time response shows arelatively fast decay.

Note that at early time ( 0 to 10-2 s) allthe curves are identical. This is becausethe secondary current system has not yetreached the lower layer.

Note that voltage is plotted on thevertical axis and is the time derivative ofthe secondary magnetic field at thereceiver (the quantity actuallymeasured).

In field data, it can be difficult to distinguish the various transients, since they all exhibit amonotonic decay over time. An alternative way to display these data is to compute theapparent resistivity (a) which is defined as follows.

)(

),( 1

1 tV

tV

obs

una =

where Vun (1, t) is the voltage decay that would be observed over a uniform earth withresistivity 1 and Vobs (t) is the measured voltage decay. To compute a this requires thatthe value of1 must be known. This can be avoided by assuming that the voltage decay isin the late-time stage and choosing =2/3.

It can then be shown that

3

22

)(5

2

4 ttV

MIr

t obsa

=


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where ris the transmitter loop radius,Mis the receiver coil moment (coil area multiplied

by the number of turns) and I is the transmitter current (Fitterman and Stewart, 1986).When the decay curves on the previous page are converted to apparent resistivity, theresults shown below are obtained.

Note that at early time (up to 10-2 s) the apparent resistivity does not equal the trueresistivity of 1 m. This is because the late time approximation was used to compute theapparent resistivity. Beyond a time of 10-2 s the apparent resistivity faithfully reflects theincrease and decrease of the true electrical resistivity for the range of models.

Additional details are described by Fitterman and Stewart, (1986)


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C6.2 Time-domain EM systems

6.2.1 Airborne EM systems with offset TX and RX

INPUT (Induced Pulse Transient System)


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one of the most successful time-domain systems is the INPUT system. Thishas been widely used in both ground and airborne surveys. The basicconcept is shown in Beck, Figure 7.33 and also Telford Figure 7-28.

note that the response is largest (more negative) over conductive targets.

successive traces correspond to later times (deeper structures). Conductiveoverburden is seen in the first few channels, but not later on. In contrast,the sulphides are consistently seen in all channels.

INPUT data can generally image deeper than corresponding frequency

domain EM data. The quiet period of recording without the primary fieldsallows secondary fields to be detected from greater depths.


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GEOTEM

-transmitter is mounted around the wings of a small aircraft (Casa 212)

-uses same half sinusoidal waveform as INPUT

MEGATEM

Deeper exploration can be achieved with a larger transmitter dipole moment. Thisproduces a stronger secondary magnetic field, which stays above the background noiselevel for a relatively long time. Since later times correspond to deeper signal penetration,this gives a greater depth of exploration. The MEGATEM system is named because it has

a transmitter dipole moment in excess of 106 Am2 formed by placing a large transmitterloop around the wings of a Dash-7.


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6.2.2 Ground-based TDEM systems

These ground-based TDEM systems can use a flexible layout and the TX size can beadjusted from 1x 1 m to 2000 x 2000 m. Larger loops can be used to boost signal strengthand give deeper signal penetration. Having the TX and RX stay in the same location for a

period of time allows stacking i.e. record many on-off cycles of the TX and add theresponses together. This allows detection of weaker signals and the removal of incoherentnoise.

The RX can be placed in the centre of the transmitter (central-loop configuration) or at avariable offset. Collecting transient data at variable offsets can give additional resistivity-depth information. It also takes advantage of the fact that it is logistically easier to move asmall RX than the larger TX.

Widely used systems include

SIROTEM (developed by CSIRO in Australia)A SIROTEM time-domain EM system

being used in a geotechnical survey inTurkey. Note the TX loop of wire onground

Crone Pulse EM (www.cronegeophysics.com/)

Geonics produce the TEM47, TEM57 and TEM67 transmitters as well as theProTEM receiver.

Images of the Geonics ProTEM system (www.geonics.com)
http://www.cronegeophysics.com/http://www.geonics.com/http://www.geonics.com/http://www.cronegeophysics.com/


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6.2.3 Time-domain EM systems for deeper exploration

Imaging to depths beyond 1-2 km withTDEM requires powerful specializedsystems. One of the most useful is the

long-offset transient electromagneticsystem (LOTEM), which generates a

powerful transient that can be detectedby an array of receivers on a profileextending away from the transmitter.The LOTEM technique can beconsidered analogous to seismicrefraction, with EM energy travellinghorizontally in the Earth. In contrast, theEM energy travels vertically inmagnetotellurics, which can be

considered analogous to seismicreflection.

The transient signal is generally quiteweak at the RX and stacking is needed toimprove the signal-to-noise ratio.

Note that both electric and magneticfields are recorded.

The LOTEM system is described byStrack et al., (1990) and an applicationto crustal scale exploration isdocumented by Hordt et al., (1992) andHordt et al., (2000).

Strack et al., (1990) showing that aconductor at 8 km depth was detectedwith LOTEM. This feature was alsoimaged with MT data.


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The UTEM system uses a large loop (up 2 x 2 km) and a unique triangular waveform(West et al., 1984) and has been applied to crustal exploration in the Kapukasing uplift(Kurtz et al., 1989).

6.2.4 Very early time EM systems

The need to map shallow structure in environmental surveys has led to the developmentof new systems that can measure transient responses at very early times. These systemsmap shallow structure and act as good metal detectors.

Since early time corresponds to high frequency, care must be taken to ensure thatdisplacement current can really be ignored from the data analysis.

Also note that it takes a finite amount of time to switch off the TX current. This canrequire special electrical engineering in very early time EM systems.

Note that shallow EM systems can detect objects that are routinely missed by magneticsurveys. This is because magnetic surveys only detect ferrous objects, while EM surveyscan potentially detect all metallic objects. This can be an important advantage insearching for UXO and landmine clearance.

Geonics EM61

The decay of the secondary field is relatively quick when the target is small. The GeonicsEM61 uses early time measurements made from 216-1266 s to detect small, shallowmetal targets, such as UXO. This system can be hand-carried, towed on a vehicle or usedunderwater..

The figure on the right shows the depth at which metal pipes of various sizes can bedetected with the EM61.

VETEM

System developed by the United States Geological Survey for Very Early TimeElectroMagnetic measurements (VETEM).


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6.2.5 Central loop airborne EM systems

As outlined above, airborne EM exploration systems have evolved in the last 50 yearsinto two basic forms:

Advantages DisadvantagesFrequency domain helicopter

EM system with rigid boom

and small TX moment

Good discrimination betweengood and bad conductors.

Good horizontal resolutionbecause of close TX and RX.

Limited depth penetration

Fixed wing time-domain

systems with towed RX and

large TX dipole moment

Good depth penetration due tostrong transmitter

Limited discriminationbetween good and badconductors.

Limited horizontal resolution.

Several attempts have been made to combine the strengths of the above systems. Onesuccessful development is the AeroTEM system that has been developed by AeroquestSurveys in Missisagua This system has a TX with a strong dipole moment (40000 Am2)that can be towed closer to the ground than the TX in a fixed wing system such asGEOTEM, giving relatively deep signal penetration. Responses can be detected at the

part-per-billion (ppb) level (Boyko et al., 2001; Balch et al., 2003). The coincident TXand RX give a sharp anomaly as the system is flown over a target. Unlike towed-birdsystems, the response is independent of flight direction. This geometry also givesmaximum coupling between ground conductors and the RX and TX loops. AeroTEMalso makes measurements during the on-time, and this allows better discrimination of the

target conductance.

The data panel shows the early time z-axis AeroTEM response for a survey near Sudbury(Balch et al., 2003). The positive anomalies (red) have a maximum amplitude of about 1

ppm and are sulphide bodies and the negative (blue) anomalies are powerlines. Thissurvey discovered a previously unknown sulphide deposit located between the two

powerlines.

Another similar system is the HeliGEOTEM system operated by Fugro Airborneservices.


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6.3 Applications of time-domain EM

6.3.1 Mineral exploration

INPUT exploration for massive sulphides

Texas Gulf Sulphur Timmins (Telford Figure 7.28). Note that the use of multiple timechannels allows shallow conductors (overburden) to be distinguished from deeperconductors (sulfides).

Helicopter MK VI INPUT data collectedover the Goldstream sulphide body (Cu,Zn, Ag). Telford Figure 7.108b. Thedashed curves shows the total magneticfield anomaly over the target.

Telford shows how simple forwardmodelling can be used to determine thedimensions of the ore body.

Helicopter MK V1 INPUT over WindyCraggy sulphide body (Cu) in theYukon. Telford Figure 7.108d


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GEOTEM exploration for kimberlites

Kimberlite pipes are often characterized by a low resistivity disk at the surface. This isproduced when weathering of the kimberlite produces a clay layer that has a lowelectrical resistivity. In northern Canada, glacial erosion often creates a lake and the clay

becomes water saturated, further lowering the resistivity. The combination of airborneEM and aeromagnetic data is widely used in current exploration.

Example : Willy Nilly Kimberlite pipe (www.fugroairborne.com.au)

GEOTEM on-time conductivity

GEOTEM off-time

Aeromagnetic anomaly
http://www.fugroairborne.com.au/http://www.fugroairborne.com.au/


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6.3.2 Hydrocarbon exploration

This example shows how paleo-channels can be located with AEM data. These channelscan host shallow gas reservoirs that depths as shallow as 50 m and are characterized aszones of high resistivity. This example is from somewhere in Alberta, and more details

can be found at www.fugroairborne.com.au by searching for Shallow gas. In typicalWCSB conditions, MEGATEM and GEOTEM can give penetration up to 300 m. Costscan be as low as $100/km for large volumes of data, which is lower than the cost ofsurface geophysics.
http://www.fugroairborne.com.au/http://www.fugroairborne.com.au/


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6.3.3 Groundwater exploration

Time domain EM exploration is widely used in the search for groundwater. The followingsynthetic examples are taken from Fitterman and Stewart (1986) and illustrate theresolution that is possible.

This example shows the late-timeapparent resistivity for a set of modelswith resistive basement at depths from12 to 400 m. Note that the highresistivities at early times are an artefact

of using the later time approximation tocompute apparent resistivity.

This set of model illustrates an example

of imaging a layer of gravel that couldpotentially be an aquifer. The resistivelayer is observed in the apparentresistivity between t = 0.01 and 0.1seconds. Note that the high resistivitiesat early times are an artefact of using thelater time approximation to computeapparent resistivity.

The following examples of real time-domain EM data from Southern California are takenfrom Taylor et al., (1992) and the transients have been converted to apparent resistivity,as described in C6.1.3. The time-domain apparent resistivity has then been used togenerate a layered Earth resistivity model. The range of possible models is also shown inthe right hand panels. The relatively rough curve in the right hand panels is a coincidentwell log, and in each case quite good agreement. The low resistivity layer is likely asaline aquifer.


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6.3.4 Geotechnical exploration

Time-domain EM study by Ersan Turkoglu in Avcilar, a suburb of Istanbul that was badlydamaged by the 1999 Izmit earthquake. SIROTEM data were used to image resistivity inupper few hundred metres of the subsurfce, and reveal possible shallow faults. UnlikeMT, this technique can be easily used in urban areas with high levels of cultural noise.

This study also used DC resistivity data, and a joint inversion of TEM and DC data toovercome some of the inherent non-uniqueness.


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References

Balch, S. J., W. P. Boyko, N. R. Paterson, The AeroTEM airborne electromagnetic system, The

Leading Edge, 562-566, 2003.

Boyko, W., N. R. Paterson, and K. Kwan, AeroTEM characteristics and field results, The LeadingEdge, 1130-1138, 2001.

Goldman, M., et al., On reducing the ambiguity in the interpretation of transient EM soundingdata, Geophysical prospecting, 42, 3, 1994.

Fitterman, D. V., and M. T. Stewart, Transient electromagnetic sounding for groundwater,Geophysics, 51, 995-1005, 1986.

Fitterman, D.V. et al., Electromagnetic mapping of buried paleochannels in eastern Abu DhabiEmirate, U.A.E., Geoexploration, 27, 111, 1991.

Fountain, D., Airborne electromagnetic systems - Fifty years of development, ExplorationGeophysics, 29, 1-11, 1998.

Hoversten, G. M. and H. F. Morrison, Transient fields of a current loop source above a layeredearth, Geophysics, 47, 1068-1077, 1982.

Hordt, A., et al., Inversion of LOTEM soundings near the borehole Munsterland-1, Germany andcomparison with MT measurements, Geophys. J. Int., 108, 930-940, 1992.

Hordt, A., S. Dautel, B. Tezkan and H. Thern, Interpretation of long-offset transientelectromagnetic data from the Odenwald area, Germany, using two-dimensional modelling,GJI, 140, 577-586, 2000.

Kurtz, R. D., J. C. Macnae and G. F. West, A controlled-source, time-domain electromagneticsurvey over an upthrust section of Archean crust in the Kapuskasing Structural zone, GJI,195-203, 1989.

Meju, M. A., Joint inversion of TEM and distorted MT soundings: some effective practical

considerations, Geophysics, 61, 56-65, 1996.Meju, M. A., A simple method of transient electromagnetic data analysis, Geophysics, 63, 405-

410, 1998.Nabighian, M. N., Quasi-static transient response of a conducting half-space- An approximate

representation, Geophysics, 44, 1700-1705, 1979.Strack, K.-M., E. Luschen, and A. W. Kotz, Long-offset transient electromagnetic (LOTEM)

depth soundings applied to crustal studies in the Black Forest and Swabian Alb, FederalRepublic of Germany, Geophysics, 55, 834-842, 1990.

Taylor, K., et al, Use of transient EM to define local hydrogeology in an arid alluvialenvironment, Geophysics, 57, 343, 1992.

Ward, S.S., Electrical prospecting with non-sinusoidal alternating currents, Geophysics, 306-314,1938.

West, G. F., J. C. Macnae, and Y. Lamontagne, A time-domain EM system measuring the stepresponse of the ground, Geophysics, 49, 1010-1026, 1984.

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